Conventional self-powered machineguns firing high-pressure bottlenecked cartridges came into common use late in the 19th century. The design of self powered machineguns and their bottlenecked cartridges have not essentially changed since their inception. Bottlenecked cartridges are required because cartridge cases must contain enough propellant to be able to provide adequate power without the cartridge cases being excessively long. Bottlenecked cartridges, by definition, are larger in diameter at the base than at the neck. The pressure area at the base of the cartridge is relatively larger than the basal area of the projectile being fired. This means there is more longitudinal force applied to the base of the cartridge than to the base of the projectile. Also, since the bodies of bottlenecked cartridge cases are larger than bore diameter, then more radial force is applied to the walls of the chamber than to any other part of the barrel. The largest diameter of the bottleneck cartridge, rather than the projectile diameter, dictates the design strength of the chamber and of the weapon locking system parts. The employment of bottlenecked cartridges results in weapon designs that are much larger and heavier than would be required than if cartridges were designed to be longer for a given propellant capacity, rather than larger in diameter.
Small diameter, high efficiency cartridges provided with conventional case heads cannot be employed in conventional small arms, however, because conventional cartridge case heads will not tolerate the higher pressures required for high efficiency conversion of propellant energy into projectile kinetic energy.
A gun, like an automobile engine, is a heat engine. The thermodynamic efficiency of converting the potential chemical energy of the propellant into kinetic energy in the projectile (other things also considered) is a function of the temperature drop across the heat engine process. Therefore the greater the temperature drop across the heat engine process the more efficient the process. Increased thermodynamic efficiency means less propellant (in a smaller cartridge case) is required to impart a given amount of kinetic energy to the projectile.
Pressure drop across the thermodynamic process equates directly to temperature drop (other things considered) as a measure of thermodynamic conversion efficiency. Therefore, the greater the operating pressure the weapon/cartridge system can tolerate, the higher the potential thermodynamic efficiency, and the smaller the required cartridge case for a given projectile weight and velocity. Smokeless powder, as used in conventional military small arms ammunition, is capable of generating about 230,000 pounds per square inch (psi). The highest normal operating pressures employed in conventional small arms weapons is 57,400 psi, or about 25% of the potential pressure of the propellant. The reason conventional cartridge cases cannot operate at higher pressure is that the rear of the cartridge case head, with its primer, must protrude from the rear of the barrel chamber in order to provide access for the weapon extractor to the extraction groove of the cartridge case. The primer is fully pressurized by the propellant gases, and while the bolt face fully supports the longitudinal pressure exerted against the base of the primer, the sole support for the radial pressure within the primer is provided by the strength of the cartridge case head itself. This means the operating pressure limit in a conventional cartridge is determined by the strength of the cartridge case itself, irrespective of the strength of the weapon.
One grain weight of the double base propellant used in the 5.56 mm NATO Cartridge employed by the U.S. Military contains about 215.15 ft/lbs. of chemical energy. The propellant charge for the 5.56 mm cartridge is about 27.0 grains. The potential energy of the propellant in this cartridge is therefore 215.15×27.0=5,809.05 ft/lbs. The muzzle velocity of a 62 grain projectile fired from a 5.56 mm cartridge is about 3,050 ft/sec, yielding a muzzle energy of about 1,280 ft/lbs. The thermodynamic conversion efficiency is therefore 5,809.05/1,280=0.2203, or about 22%.
Conventional, high pressure, full power machineguns are provided with some means for locking the cartridge within the weapon barrel chamber during firing. The locking system is typically composed of complex and tightly toleranced parts. The locking system parts interact with each other during firing to sequentially perform the steps in the operating cycle. Locking, firing, extraction and ejection functions are typically concentrated in a small volume at the front of the bolt, which means the functions and the parts involved compete for space.
In conventional weapon and cartridge design there is usually a gap between the face of the fully locked weapon bolt and the rear of the cartridge. This gap is not desirable, but is the result of weapon and ammunition manufacturing tolerances, as well as weapon wear. This gap results in what is called “headspace.” This actual headspace must be accounted for in the design of the weapon and its ammunition, even though ideally there would be zero headspace. Zero headspace would place the cartridge in intimate contact with the face of the bolt for firing. Even with zero headspace there is always some elastic deformation of the weapon locking system parts permitting some elastic movement of the cartridge case head during firing.
When a high pressure cartridge is fired, the firing pressure forces the cartridge case wall against the wall of the chamber, and at the same time the firing pressure also drives the head of the cartridge rearward. The cartridge case wall adjacent to the cartridge case head stretches elastically and then plastically rearward while the body of the case is seized within the chamber. If there is excessive headspace, the cartridge case wall adjacent to the cartridge case head will stretch plastically until the cartridge case head is weakened. At some point, this plastic stretching can result in separation of the cartridge case head from the case body, resulting at worst, in the release of large amounts of high pressure gas into the weapon breach, blowing up the weapon.
The employment of conventional high-pressure bottleneck cartridges in conventional small arms weapons has resulted in relatively heavy, inefficient and expensive machineguns and ammunition.